Apparatus and method for producing holograms with acoustic waves



Sept. 16, 1969 APPARATUS Filed June 30. 1967 G. A. MASSEY 3,467,216 AND METHOD FOR PRODUCING HOLOGRAMS WITH ACOUSTIC WAVES 5 Sheets-Sheet 1 SIGNAL '4 7/ GENERATOR A DRIVE l6 MECHANISM 20 E W NARROW B 3%? I 4 REFLECTING OBJECT INVENTOR- GAIL A. MASSEY ATTORNEY Sept. 16, 1969 Filed June 50, 1967 a. A. MASSEY 3,467,216 APPARATUS AND METHOD FOR PRODUCING HOLOGRAMS WITH ACOUSTIC WAVES 5 Sheets-Sheet 2 SIGNAL 4Q GENERATOR 39 l L t f 37 36 fiwi'fi gggg eg,$UMMlNG FILTER AMPLIFIER 138 34 [35 B REFLECTING 29 OBJECT l mvsu'ron B I GAlL A. MASSEY BY M J ATTORNEY Sept. 16, 1969 a. A. MASSEY 3,467,216

APPARATUS AND METHOD FOR PRODUCING HOLOGRAMS WITH ACOUSTIC WAVES Filed June 50, 1967 5 Sheets-Sheet 5 A COMBINING B Y NETWORK SIGNAL GENERATOR gm omvs. 36 Z MECHANISM 34' 37 NARROW BAND FILTER COMBINING T NETWORK VARIABLE v 38 1 PHASE & SHIFTER 39 SIGNAL \68 GENERATOR INVENTOR- GAIL A. MASSEY ATTORNEY Sept. 15, e. A. MASSEY 3,467,216

APPARATUS AND METHOD FOR PRODUCING HOLOGRAMS WITH ACOUSTIC WAVES Filed June 30, 1967 5 Sheets-Sheet 4 INVENTOR.

GAIL A. MASSEY ATTORNEY Sept. 16, 1969 ca. A. MASSEY APPARATUS AND METHOD FOR PRODUCING HOLOGRAMS WITH ACOUSTIC WAVES 5 Sheets-Sheet 5 Filed June 30, 1967 INVENTOR GAIL A. MASSEY TTORNEY States ABSTRACT OF THE DISCLOSURE A loudspeaker and a microphone are spaced from a reflecting object and from each other. The microphone transcribes a raster-type scan in a plane that is inclined With respect to a line through the centers of the reflecting object and the scanned area. The output of a signal generator causes the loudspeaker to illuminate the object with acoustic Waves. These waves are reflected by the object and are detected by the scanning microphone which produces an output that is combined with the output of the signal generator in a summing network. The combined output signals intensity modulate a lamp which moves with the microphone. The field pattern defined by variations of the light intensity of the lamp is recorded on the photographic plate of a camera. The plate is parallel to the plane area scanned by the microphone. This recording is a hologram of the reflecting object. A visual image of the object is produced by illuminating the developed photographic plate (the hologram transparency) with coherent light from a laser.

BACKGROUND OF THE INVENTION This invention relates to holography and more particularly to apparatus for and a method of utilizing acoustic signals for producing holograms.

A hologram is a recording of the Fresnel diffraction pattern of an object which may be used to produce a three-dimensional image when the hologram is illuminated with a coherent light signal. Techniques utilizing coherent light generated by a laser are presently available for producing holograms. In certain applications, however, it is desirable to produce an image of objects located in a medium that is substantially opaque to light and other electromagnetic radiation. Such applications include underwater mapping, geological exploration, and imaging SUMMARY OF INVENTION In accordance with this invention, an electroacoustie transducer is energized by an electrical signal from a signal source to produce acoustic signals which illuminate a reflecting object. Acoustic signals reflected by the object are detected by a plane detector assembly to produce an electrical signal that is combined with an electrical reference signal. The combined signal is processed and recorded to produce a hologram of the object. In one embodiment of the invention the reference signal is obtained directly from the signal source. In a modified form of this invention, acoustic waves reflected by the object are detected in a am E Patented Sept. 16, 1969 plane that is inclined with respect to the direction of propagation of waves from the object, i.e., the detection plane forms an acute angle with the line of propagation of the reflected waves.

DESCRIPTION OF DRAWINGS This invention will be more fully understood from the following detailed description of preferred embodiments thereof, together with the accompanying drawings in which:

FIGURE 1 is a plan view of a system embodying this invention;

FIGURE 2 is a plan view of a system embodying a modified form of this invention;

FIGURE 3 is a graphic representation of the orientation of the detection plane of the detector assembly;

FIGURE 4 is a perspective view of equipment embodying the system of FIGURE 2;

FIGURE 5 is a plan view of part of the detector assembly shown in FIGURE 4;

FIGURE 6 is an enlarged section of the horizontal boom and detector assembly taken along line -66 in FIGURE 5;

FIGURE 7 is a perspective view of an alternate embodiment of the detector assembly;

FIGURE 8 is a photograph of a hologram of the reflecting objects shown in FIGURE 4;

FIGURE 9 is a photograph of the reconstructed images of the objects when the hologram of FIGURE 8 is illuminated by coherent light from a laser;

FIGURE 10 is a drawing illustrating the true shape and arrangement of the reflecting objects which produced the images shown in FIGURE 9; and

FIGURES 11 and 12 are plan views of other systems embodying modified forms of this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to FIGURE 1, a system embodying this invention comprises an acoustic transmitter 1, an acoustic reference transmitter 2, and a detector assembly 3 which are located in a medium 4 that is capable of propagating acoustic or sound waves. The invention may be practiced with a gaseous medium 4, such as air, that is capable of propagating both acoustic and electromagnetic waves, or with a solid medium such as metal (see FIGURE 12), or a liquid medium, such as water, that support acoustic waves but which are opaque or substantially so to electromagnetic waves.

Acoustic transmitters 1 and 2 are energized by sinusoidally varying electrical signals on lines 5 and 6, respectively, which are produced by signal generator 7. Each transmitter comprises an electroacoustic transducer or loudspeaker. Transmitter 1 is positioned such that sound waves 8 generated thereby are directed toward and strike reflecting object 9 which is also located in the medium 4. Detector assembly 3 is positioned to receive sound waves 8' reflected by the object. Detector assembly 3 is laterally offset from transmitter 1 and the direction of sound wave propagation along line BB intersects the line AA from object 9 to the center of assembly 3 to form an angle 0:.

Reference transmitter 2 is adjacent to object 9 and is oriented so as to propagate reference waves along line E--E toward detection plane DD of assembly 3. Line EE through the centers of transmitter 2 and assembly 3 intersects line A-A at an angle 5.

The detector assembly comprises an electroacoustic transducer or microphone 11 facing the reflecting object and rigidly secured to a support plate 12. Microphone 11 preferably has a linear frequency response and is oriented such that its longitudinal or focal axis is parallel to line A-A. Plate 12 is operatively connected to drive mechanism 14 which causes the microphone to transcribe a rastertype scan in the detection plane DD over an area such as M N O P (see FIGURE 4) to detect sound waves reflected from the object. The output of the microphone on line 15 is amplified by amplifier 16 and filtered by narrow band filter 17. The center frequency of the filter is equal to the frequency of the output of generator 7, i.e., the frequency of the acoustic signal. The output of the filter is applied to lamp 18 which is also rigidly secured to and moves with plate 12. The intensity of light from the lamp is preferably a linear function of input power applied to it.

Camera 19 is spaced from the detector assembly and is positioned so that the area scanned by the lamp is within the field of view of the camera. Preferably, the camera and lamp are so positioned that the focal axis of the camera is normal to the plane of movement of the lamp, i.e., the camera axis lies along the line A-A. Photographic plate 20 on which the hologram is recorded is therefore parallel to detection plane DD.

In operation, the output of signal generator 7 causes loudspeaker 1 to transmit acoustic waves 8 to the reflecting object 9. Certain of those waves 8 are reflected by the object back to detection plane DD. Reference transmitter 2 is responsive to the output of generator 7 for producing sound waves 10 which are also incident on detection plane DD. Microphone 11 linearly combines or adds refiected waves 8 and reference waves 10 and produces an electrical signal on line 15 corresponding to the sum. The output of filter 17 intensity modulates lamp 18 and produces an optical field which is the analog of the corresponding acoustic field, resulting in an optical hologram field of the object. Driven by mechanism 14, the microphone traverses a raster-type scan in the plan DD and produces a hologram field wholly representative of the object. Camera 19 records a hologram of object 9 on photographic plate 20 which is exposed to the optical field produced by lamp 18. The nonlinear film response in effect causes the analogs of the reference and reflected waves to be mixed and the recorded trace on the film corresponds to the detected pattern of the reflecting object. An image of the object is produced in the well known manner by illuminating the hologram on plate 20 with coherent light from a laser.

In the system of FIGURE 1, the reference transmitter is located in the medium containing the reflecting object. Although placement of a reference transmitter in the medium may be readily accomplished in a gas, such as air, and in a controlled environment, this may be impracticable where the medium is a solid or solid particles, such as soil, or a liquid such as water, and the reflecting object is inaccessible or unknown. A modified form of this invention which does not require an acoustic reference transmitter is illustrated in FIGURE 2.

Referring now to FIGURE 2, the system comprises an acoustic transmitter 21, a detector assembly 23, and a signal generator 27. Transmitter 21 and generator 27 are the same as the transmitter 1 and generator 7, respectively, of the system of FIGURE 1.

Detector assembly 23 is similar to detector assembly 3. Microphone 31 and lamp 32 are rigidly secured to the same side of support plate 33. Drive mechanism 34 is operatively connected to plate 33 for causing the latter to transcribe a raster-type scan in the detection plane D'D', and the longitudinal or focal axis of microphone 31 is parallel to the line A'A' from the object to the center of detector assembly 23 during movement of the microphone. Plane D'D', however, is inclined at an angle 6 with respect to line A'A'. This angular inclination of the detection plane provides for spatial separation of the real, virtual, and zero-order images in the reconstruction process. The spatial frequency of a field that varies sinusoidally in amplitude along a given direction in a detection plane is the reciprocal of the distance corresponding to the spatial period of the variation. For propagating waves, changing the relative angle of arrival of the wave at the detection plane changes the spatial period and therefore the spatial frequency.

Referring now to FIGURE 3, the spatial frequency f of acoustic waves incident on detection plane D'- D at the angle 6 is representable as i l/A sin '7 wherein A is the wavelength of the acoustic waves and 'y is the angle between detection plane D'-D and line A'A'.

Generally, a complex field can be described by a spectrum of spatial frequencies corresponding to a set of waves arriving at different angles. If all the angles lie within a cone with an apex half-angle less than 90, this spectrum is said to be band limited. Such a spatial signal can be frequency translated by changing the average angle of arrival relative to the detection plane. If the original cone were centered about the normal to the plane, spatial translation to a new angle 0 would correspond to imposing the old variation as a spatial envelope modulation on a new carrier of spatial frequency f=sin 0/1, where is the wavelength. When spatial frequency translation is effected in recording a hologram, the diffraction components produced in the reconstruction process are therefore angularly displaced relative to each other; thus, the real and virtual images, as well as the zero order beam, do not overlap.

The output of microphone 31 on line 35 is ,tamplified by amplifier 36 and filtered by narrow band'filter 37. The bandwidth of filter 37 is centered at the frequency of the output of transmitter 21. The output of filter 37 is applied on line 38 to combining network 397 The output of generator 27 is also applied on line 40 to the second input to network 39. The combining network may, by way of example, be a summing amplifier. The output of network 39 is amplified by amplifier 41 and is applied to lamp 32.

Camera 42 is also spaced from the detection plane with its lens facing lamp 27. The longitudinal or focal axis of camera 42 preferably extends in a direction perpendicular to detection plane D'-D and intersects line A'A at the angle Photographic plate 43 (the hologram recording plane) is parallel to the detection plane D' In operation, the output of generator 27 causes transmitter 21 to illuminate the reflecting object with acoustic waves. Reflected sound waves incident on assembly 23 are detected by microphone 31. The detected signals are then combined in network 39 with the electrical reference signal on line 40. The output of network 39 is applied to lamp 32 and intensity modulates it to provide a light signal representative of the acoustic field of the reflecting object. Mechanism 34 drives the microphone and lamp in a raster-type scan over detection plane D' D. The lamp exposes plate 43 and makes a permanent record or hologram of the fields produced by the refleeting object. As before, an image of the object is reconstructed by illuminating the hologram with coherent light.

Equipment embodying the system of FIGURE 2 and which was actually built and tested is illustrated in FIG- URE 4. Transmitter 21 comprises loudspeaker 46 located at the focal point of parabolic reflector 47. A metallic cone 48 connected to loudspeaker 46 directs acoustic signals from the latter to illuminate reflector 47. The aperture of the reflector faces reflecting objects 29a-e.

Detector assembly 23 and drive mechanism 34 are supported on opposite ends of horizontal boom 51 which is connected for vertical movement to column 52. Such movement of boom 51 is controlled by drive motor 53 operatively connected to the boom by flexible drive cable 54 which extends through column 52. Horizontal movement of assembly 23 is controlled by drive motor 55 which is connected to the detector assembly by tape 56. Servo 57 controls the operation of motors 53 and 55 to synchronize vertical and horizontal movement 'of the parts and produce the desired raster scan of the microphone and lamp.

Referring now to FIGURES 5 and 6, detector assembly 23 comprises parabolic reflector 58 connected to plate 33. Microphone 31 is supported at the focal point of reflector 58. The microphone faces reflector 58 and received signals reflected from it. Lamp 32 is also supported by plate 33 and is located above the microphone and reflector. Plate 33 is connected to guide 59. Guide bars 60 extend through and support the guide above boom 51. The guide bars are suspended between stop plates 61 and 62 which are connected to opposite ends of boom 51. Tape 56 extends through and is secured to guide 59.

Boom 51 is positioned such that the longitudinal axis VV thereof is at an acute angle with respect to the plane W-W containing objects 29. The focal axis XX of microphone reflector 58, however, is perpendicular to the plane W-W. The optical axis Z-Z of camera 42 is perpendicular to the plane of movement of lamp 32 which is parallel to the longitudinal axis VV of the boom.

Servo 57 causes the detector assembly (microphone 31, reflector 58, and lamp 32) to traverse a raster-type scan. Motor 55 causes microphone 31 to scan horizontally, for example, from a point on line M P to a point on line N see FIGURE 4-. When guide 59 contacts a microswitch (not shown) at plate 61, the servo causes motor 53 to incrementally shift the vertical position of boom 51. The servo then reverses the polarity of the signal applied to motor 55 to scan the microphone from a point on line N 0 to a point on line M P This operation is repeated until the microphone transcribes the area M N O P Alternatively, the lamp 32 may be electrically disconnected from the combining circuit and the polarity of the signal applied to motor 55 reversed to move the microphone back to the line M 1 before the motor 53 is energized to change the vertical position of the boom 51. Since lamp 32 is offset from microphone 31 and is also connected to plate 33, the lamp scans an area M N O P similar to that scanned by the microphone. The intensity of light from the lamp at each point in the scanned area corresponds to the characteristics of the signal received by the microphone. Thus, lamp 32 produces an optical field which is the analog of the acoustic field produced by the reflecting objects.

Alternatively, the detector assembly may be a fixed array 63, see FIGURE 7, comprising a plurality of individual microphones 64. The microphones are arranged in rows that extend over the area M N O P and thus acoustically sensitize it. The signal detected by each microphone is correlated with its position in the array and is combined with the electric reference signal for application to an associated lamp 65 to produce the hologram of the objects.

In laser holography, optical detection by photographic film is approximately a square law phenomenon and therefore requires a laser reference beam to determine the relative phase of the reflected and reference signals. As disclosed by E. N. Leith and J. Upatneiks, Journal of the Optical Society of America, vol. 52, N0. 10, pp. 1123- 1130, October 1962, however, the laser reference beam is offset from the laser beam reflected by the object to provide spatial frequency translation of the zero order and virtual and real images of the objects. This technique is employed in the system of FIGURE 1 wherein the acoustic reference beam (line EE) makes an angle B with the normal AA to the detection plane DD.

It has been discovered that in acoustic holography, however, the signals may be separately or individually linearly detected and then combined to produce a signal representative of the incident acoustic field. Thus, an electric reference signal, in place of an acoustic reference sig nal, may be combined with the detected signal reflected 6 from an object. Such a system is illustrated in FIGURE 2. In order to provide for separation of the diffracted images in the reconstruction process, the detection plane D'D' of the system of FIGURE 2 is inclined with respect to the line A'A'. Referring now to FIGURE 2, the detected signal from microphone 31 is representable as where U (x,y) is the acoustic field that would be present at the detection plane DD' if the incline angle were zero, K is the transfer characteristic of microphone 31, 6 0 is the variation of the acoustic field introduced by inclining the detection plane D'D' and f is the spatial frequency introduced in the scanning plane by the tilt. For simplicity, the exponential term representing the sinusoidal time variation of the output of transmitter 21 is omitted from these equations. Referring now to FIGURE 3, f is the spatial frequency produced by inclining the detection plane by the angle 'y=sinf with respect to the normal to the line A'-A', wherein a is the wavelength of sound waves produced by loudspeaker 21.

The reference signal on line 40 from generator 27 (FIGURE 2) is a sinusoidal time-varying signal having a complex amplitude A. If network 39 is a summing amplifier, the power output thereof is representable as If the lamp intensity varies linearly with input power, I(x,y), the optical intensity used to expose the film, is then proportional to P(x,y). When the developed holo- 'gram transparency is exposed to a plane wave of monochromatic light, the transmitted optical field amplitude is also proportioned to P(x,y) in Equation 3. In that case, the first term of Equation 3 defines a group of transmitted waves propagating along the normal to the hologram axis along the direction of the incident wave. The second and third terms of Equation 3 define groups of waves propagating off the axis and along or centered about the angles i sin f away from the normal. These terms represent the transmitted waves which produce the spatially separated virtual and real images, respectively, of the reflecting object. If the inclination of the detection plane corresponding to the constant f were not present, all the waves would be on the axis and the images would overlap.

By way of example, FIGURE 8 is a photograph of a hologram of objects 29a-e produced by the system of FIGURE 4 which had the following dimensions, components and characteristics:

Reflecting objects:

Shape Octagonal. Material Aluminum. Width 15 cms. 292

Shape Square. Material Aluminum. Width 19 cms. Spacing (objects 29 to plane D'D'). 5 meters. Scan area M N O P 1 sq. meter. Raster spacing 1 cm. Sound waves 8 wavelength 1 cm. Average sound level (at plane D'D). 0.05 microbars. Medium Air.

In order to reduce reflection of sound waves from the walls adjacent the objects, sheets of foam rubber (not shown) were placed behind the objects. The hologram of the objects was recorded on the photographic plate 43 in the manner described above. The film was then photographically reproduced to a positive transparency 0.5 cm. square. This transparency was illuminated by a laser beam operating at 6328 A. FIGURE 9 is a photograph of reconstructed images of the objects produced by illuminating the hologram with a laser beam. FIGURE 10 is a scale drawing of the objects 29 that are illustrated in FIG- URE 9. A modified form of the invention providing for spatial frequency translation of the zero order and real and virtual images of the objects and wherein the detection plane DD is normal to the center line A-A of the detector assembly is illustrated in FIGURE 11. This system is similar to the system of FIGURE 2 except that the inclination of detection plane DD is zero and variable phase shifter 68 is connected in series between the output of generator 27 and the associated input of network 39. An output of the drive mechanism 34' on line 69 is applied to and controls the operation of the phase shifter. This bias signal causes the phase shifter to vary the phase of the reference signal on line 40 an amount corresponding to the term E O which is representative of the spatial frequency translation produced by tilting the detection plane D'D in the system of FIGURE 2.

The systems described above operated in air, a fluid which propagates both acoustic and electromagnetic waves. This invention is not limited to operation in such a medium, however, and operates in association with media such as the earth or metal walls which propagate acoustic waves more effectively and efficiently than electromagnetic waves. This invention also operates in association with a liquid fluid such as water which propagates acoustic waves. The system of FIGURE 12 is similar to the system of FIGURE 2 except that a medium 71 that is substantially opaque to electromagnetic waves but which propagates acoustic waves is spaced between the reflecting object 29 and the acoustic transmitter 72 and detector assembly 7 3.

The medium 71 may, by way of example, be a metal wall. Transmitter 72 comprises a piezoelectric loudspeaker which is bonded to the surface of the wall. Detector assembly 73 is similar to the assembly 65 of FIGURE 7. Microphones 74 are piezoelectric microphones that are bonded to the surface of the wall. The lamps 75 are located on the opposite side of support block 76 from the microphones and face camera 42.

The medium 71 may also be water which is substantially opaque to electromagnetic radiation and contains a reflecting object 29'. Transmitters 72 and microphones 73 are then subaqueous devices that are located in the water. The thickness of block 76 is preferably sufiicient to posi tion the lamps 75 outside of the water.

Although this invention is described in relation to preferred embodiments thereof, changes, improvements, and modifications thereof will 'be apparent to those skilled in the art without departing from the spirit of the invention.

What is claimed is:

1. A system utilizing acoustic waves for making a hologram of an object comprising an electric signal generator,

an acoustic wave transmitter energized by said generator and oriented to direct acoustic waves toward said object whereby said waves are reflected from said object,

an acoustic wave propagating medium between said transmitter and said object detecting means responsive to said reflected waves over a predetermined field for producing electrical signals corresponding to the pattern of said waves in said field,

said predetermined field being in a plane which forms an acut angle with a line from the object to the center of the field,

8 said detecting means comprising a detector assembly oriented generally parallel to said field and coextensive therewith, means for combining the outputs of said detecting means and said generator, and

means responsive to the outputs of said combining means for producing a hologram of the object.

2. The system according to claim 1 in which said hologram producing means comprises a lamp connected to the output of said combining means and producing light which varies in intensity with changes in the output of said combining means, and

means for permanently recording the variations in the intensity of said light.

3. The system according to claim 1 in which said detector assembly comprises a microphone, and

means for moving said microphone in a raster-type scan over said field.

4. The system according to claim 3 in which said recording means comprises a lamp supported for movement with said microphone.

5. The system according to claim 4 with a permanent recording means comprising a light sensitive plate in a plane parallel to the plane of movement of said lamp.

6. The system according to claim 1 in which said detector assembly comprises an array of microphones in the plane of and coextensive with said field and said hologram producing means comprises a plane array of lamps responsive to the outputs, re-

spectively, of said microphones for producing light, the intensity of light from each lamp being proportional to the output of the microphone associated therewith, and

a plane photographic plate disposed parallel to the plane of the lamp array.

7. A system utilizing acoustic waves for making a hologram of an object comprising an electric signal generator,

an acoustic wave transmitter energized by said generator and oriented to direct acoustic waves toward said object whereby said waves are reflected from said object,

an acoustic wave propagating medium between said transmitter and said object,

detecting means responsive to said reflected waves over a predetermined field for producing electrical signals corresponding to the pattern of said waves in said field, said detecting means comprising a microphone, and

means for moving said microphone in a raster-type scan over said field,

means responsive to an output of said generator and an output of said microphone moving means for shifting the phase of the output signal from said signal generator,

means for combining the outputs of said detecting means and said phase shifting means, and

means responsive to the outputs of said combining means for producing a hologram of the object.

8. The method of making a hologram of an object with acoustic waves consisting of the steps of energizing an acoustic wave transmitter with the output of a reference generator,

illuminating the object with acoustic waves from said transmitter,

detecting acoustic waves reflected by said object over a plane field transverse to the direction of propagation of the reflected waves by moving a microphone in a raster type scan over a plane disposed at an References Cited UNITED STATES PATENTS 3,400,363 9/1968 Silverman 340-3 3,284,799 11/1966 Ross.

10 OTHER REFERENCES Fishlock, Sound in 3-D, New Scientist, Dec. 8, 1966, p. 562.

Greguss, Techniques and Information Content of Sonoholograms, The Journal of Photographic Science, vol. 14, 1966, pp. 329-332.

Mueller and Sheridon, Sound Holograms and Optical Reconstruction, Applied Physics Letters, vol. 9, No. 9, Nov. 1, 1966, pp. 328-329.

Preston and Krcuzer, Ultrasonic Imaging Using a Synthetic Holographic Technique, Applied Physics Letters, vol. 10, No. 5, Mar. 1, 1967, pp. 150152.

BENJAMIN A. BORCHELT, Primary Examiner G. H. GLANZMAN, Assistant Examiner US. Cl. X.R. 340-5; 3S03.5 

